A computer font is an electronic data file containing a set of glyphs, characters or symbols. Some fonts are scalable, such as outline fonts. Outline fonts use Bézier curves, drawing instructions and mathematical formulae to describe each glyph. Outline fonts can be resized using a single font and substituting different measurements for components of each glyph. The outline can be rendered to a bitmap for display on a computer screen. Fonts for display on a computer screen are commonly referred to as screen fonts and can be designed using a font editor. Screen fonts can be monospaced, such that every character is plotted a constant distance from a previous adjacent character while drawing. Fonts can also be proportional, such that each character has its own width. Kerning is the process of adjusting the spacing between characters in a proportional font. Tracking adjusts spacing uniformly over a range of characters. Leading refers to the distance between baselines of text. Text that is displayed on a computer screen without leading can be difficult to read due to a lack of white space between the lines of text.